Air district inspectors gathered data from the fence line and from air quality monitors at the refinery, and preliminary indications showed a sulfur release during the first flaring process early Wednesday morning.

A Chevron spokesman confirmed the refinery had a flaring incident about midnight Wednesday, but said an investigation showed it was not the source of the odor. Flaring is the process of burning off excess gas.

Officials said the rotten-egg smell was also reported by residents in Richmond.

“Once we determine the source, we will pursue any and all enforcement actions available,” said Jack Broadbent, executive director of the air district.

Other potential sources of the smell that the air district are investigating include ships, local landfills and wastewater treatment facilities, officials said.

PG&E inspectors found no leaks and no trace of natural gas in the air, Guidi said.